<?php

class Attributes
{
    public function get_abstract()
    {
        return $this->abstract;
    }

    public function set_abstract($abstract)
    {
        if(is_bool($abstract))
        {
            $this->abstract = $abstract;
        }
        else
        {
            throw new InvalidArgumentException(
                'The value is not boolean.');
        }
    }

    protected $abstract = false;
}

?>
